1. Field of the Invention
The invention relates generally to optical waveguide grating devices, and more particularly to a device that simultaneously supports more than one frequency band with accurate channel spacing.
2. Description of Related Art
Computer and communication systems place an ever-increasing demand upon communication link bandwidths. It is generally known that optical fibers offer a much higher bandwidth than conventional coaxial links. Further, a single optical channel in a fiber waveguide uses a small fraction of the available bandwidth of the fiber. In wavelength division multiplexed (WDM) optical communication systems, multiple optical wavelength carriers transmit independent communication channels along a single optical fiber. By transmitting several channels at different wavelengths into one fiber, the bandwidth capability of an optical fiber is efficiently utilized.
Fiber-optic multiplexing and demultiplexing have been accomplished using an arrayed waveguide grating (AWG) device. An AWG is a planar structure comprising an array of waveguides disposed between input and output couplers and arranged side-by-side with each other, and which together act like a diffraction grating in a spectrometer. Each of the waveguides differs in length with respect to its nearest neighbor by a predetermined fixed amount. The outputs of the output coupler form the outputs of the multiplexing and demultiplexing device. In operation, when a plurality of separate and distinct wavelengths are applied to separate and distinct input ports of the device, they are combined and are transmitted to an output port. The same device may also perform a demultiplexing function in which a plurality of input wavelengths on one input port of the apparatus, are separated from each other and directed to predetermined different ones of the output ports. AWGs can also perform a routing function, in which signals arrive on multiple input ports and are routed to multiple different output ports in accordance with a predefined mapping. The construction and operation of such AWGs is well known in the art. See for example, “PHASAR-based WDM-Devices: Principles, Design and Applications”, M K Smit, IEEE Journal of Selected Topics in Quantum Electronics Vol. 2, No. 2, June 1996; U.S. Pat. Nos. 5,002,350; 7,397,986; 7,492,991; and WO97/23969, all incorporated by reference herein.
AWGs are often used in WDM-PON (wavelength division multiplexing passive optical network) systems. A typical PON has an optical line terminal (OLT) at the service provider's central office and a number of optical network units (ONUs) near end users. Each OLT and ONU includes one or more AWGs, so that multiple channels on different transmission frequencies can be carried on a single fiber. The AWGs multiplex the channels from multiple inputs at one end, and demultiplex them into multiple outputs at the other end. In order to permit such systems to be deployed with AWGs from a variety of vendors, several wavelength “bands” have been defined, and the channel spacing within each band has also been defined. These definitions have been incorporated into standards mainly under the auspices of the International Telecommunications Union (ITU). The following spectral bands have been defined in ITU specifications:
Classification of spectral bandsBandDescriptorRange [nm]O-bandOriginal1260 to 1360E-bandExtended1360 to 1460S-bandShort wavelength1460 to 1530C-bandConventional/Center1530 to 1565L-bandLong wavelength1565 to 1625U-bandUltra long wavelength1625 to 1675The channel plans specified for three of these bands (L-, C- and S-Bands) are set forth in FIG. 1. It can be seen that channels are defined at 50 GHz intervals, though a typical system would use channels at 100 GHz spacing: either on multiples of 100 GHz or at a 50 GHz offset from multiples of 100 GHz.
Though the ITU grid shows as many as 50 channels usable in each band, for some applications it is desirable to carry channels in two or more bands. For example, it is often proposed in literature to use AWGs in a bidirectional transmission system, with one band of frequencies for upstream traffic, from ONU to OLT, and another band of frequencies for downstream traffic, from OLT to ONU. This can be achieved by using two separate AWGs on each end of the transmission path, one designed for each of the two bands. The multiplexed outputs of the two AWGs on one end are either carried separately to the other end on two separate fibers, or are combined onto a single fiber using a WDM filter/combiner. However, it would be preferable if a single AWG could suffice on each end. It is possible to use a single AWG to multiplex/demultiplex signals in more than one band, but a number of problems arise.
First, in a conventional AWG, the channel spacing is proportional to the order addressed by the AWG. For example, designs have been made which can be deployed in both the C-band and L-band using order 38 and order 37 respectively. But if the design addresses the ITU-grid with channel spacing of 100 GHz using AWG-order 38 in the C-band, then the same AWG would have a channel spacing of 37/38*100=97.4 GHz, operating on order 37, in the L-band. It would deviate from the ITU specification and therefore be incompatible with AWGs on the far end of the transmission path that are designed for the standard 100 GHz spacing.
It has consequently been proposed to change the ITU specification to allow an off-grid channel spacing for the second band. But this creates another problem in that it is difficult to standardize the second channel plan. This is due to the fact that the two bands of frequencies, in particular the separation between the two bands, is defined by technology parameters used in the manufacture of the AWG, such as the material and waveguide dispersion, which vary from manufacturer to manufacturer.
To illustrate this point, FIG. 2 is a table illustrating the channel plans of two different AWGs, both using Silica on Silicon technologies but slightly different doping levels. One technology achieves a 0.7% contrast between core and cladding and the other achieves a 1.5% index-contrast between core and cladding. The 0.7% contrast technology has waveguides with a 6×6 um core and the 1.5% technology has waveguides which are 4 um square. For these two technologies, for wavelength around 1550 nm, the sum of material and waveguide dispersion are given by
            ⅆ      N              ⅆ      λ        =                    -        0.020            ⁢      μ      ⁢                          ⁢              m                  -          1                    ⁢                          ⁢      and      ⁢                          ⁢                        ⅆ          N                          ⅆ          λ                      =                  -        0.015            ⁢                          ⁢      μ      ⁢                          ⁢              m                  -          1                    for high and low contrast respectively. All frequencies in the table are in THz.
It can be seen that both AWGs are designed for 100 GHz on-grid channel spacing on order 38 in the C-band. But the resulting L-Band channel plans are different. The L-band channels for the higher contrast technology is 24 GHz higher than for low contrast technology. Therefore a WDM-PON AWG made in one technology will not be compatible in the L-band with a WDM-PON AWG made in the other technology. This is one of the reasons why it has been difficult to define an industry standard for the channel plan for WDM-PON telecom systems. Furthermore, even if all manufacturers can agree on one off-grid channel plan for the second frequency band, that plan will soon become obsolete as future improvements in manufacturing technologies dictate still different channel plans for the second channel.